1. Field of the Invention
The present invention relates to a continuous fluidized bed granulating apparatus for continuously producing a granulated product by continuously supplying a powder material such as powdered chemical, food, industrial material, fertilizer or animal feed into a fluidized bed to fluidize it and spray adding a suitable binding agent, (any additive may sometime be contained).
2. Description of the Prior Art
Continuous fluidized-bed granulating apparatus of the type in which a granulated product is continuously introduced in a substantially constant amount at a time into a classifying section from the bottom or bottom side wall of a fluidizing chamber and the granulated product is classified by classifying means thereby selecting and delivering the granules granulated to particle sizes greater than the desired size and returning ungranulated material to the fluidizing chamber, are themselves well known in the art as disclosed for example in U.S. Pat. No. 3,880,968. However, the conventional continuous fluidizing bed granulating apparatus of this type is so designed that the charging rate of a powder material to be granulated, binding liquid feed rate, hot air temperature, etc. are determined by experiments or instinct or alternatively they are separately subjected to constant value controls in accordance with preset values based on the results of granulation tests effected preliminarity on a pilot plant. Also, it is the usual practice to maintain the charged quantity constant so that the pressure drop of the fluidized bed is detected and the rate of delivery is adjusted so as to maintain the pressure drop at a set value thereby controlling such that the amount of retention or the average retention time of the powder material to be granulated in the fluidized bed is maintained constant, and also the granulated material introduced into the classifying section from the lower part or lower side wall of the fluidizing chamber is divided into a product and fine powder smaller than a given particle size by classifying means arranged at the delivery side of fluidized bed, and the fine powder is usually returned into the raw material (the powder material to be granulated).
The above-mentioned conventional continuous fluidized-bed granulating apparatus has the following disadvantages.
(1) Even if the constant value control loops provided for the respective process variables are controlled at the predetermined set values, the moisture content of the fluidized bed is varied by such disturbances due to the variations in the moisture content and particle size of a charged powder material to be granulated as well as the variation of the heat dissipation loss due to outside air temperature changes, the variation in the evaporation rate due to outside air humidity changes and this causes variations in the rate of granulation, the bulk density of product, the state of fluidization, etc., which are closely related to the bed moisture content thereby deteriorating the quality of the product and the operational stability.
(2) When it is desired to change the particle size, bulk density or throughput of product, it is necessary to reestablish the set values of the control loops predetermined separately for the respective process variables such as the hot air temperature, fluidizing air velocity, binding liquid feed rate, etc., and therefore to change the operating conditions requires extremely complicated operations, such as, preliminarily determining their correlations in terms of trial values by granulation experiments, experientially determining set values to make readjustments on the basis of the results of such set values and so on.
(3) Where the liquid/powder ratio must be regulated from the quality or formulation of product point of view, the amount of the under sieve classified by the classifying means, i.e., the amount of fine particles smaller than a given particle size varies at all times and they are returned and mixed with the raw powder material to be granulated, thereby making it impossible to ensure that the liquid/powder ratio be maintained at the specified value.